This invention relates to electrical multi-stage filter circuits and, more particularly, to inductor-capacitor filters employing integrated transformer-inductor magnetic components.
In electrical power conversion or power supply circuits, it is often necessary to perform high-speed switching operations on the power signal to make it easier to employ magnetic circuit components such as transformers for the proper conditioning of the power signal. In the process of the high-speed switching to accomplish this conditioning, undesirable high frequency and harmonic components are introduced into the power signal. Since these high frequency components may cause radiated electromagnetic interference, it is desirable to remove such components from the power signal. This is usually accomplished using a low pass filter.
Electronic power inverters such as those described above are particularly useful in powering arc discharge lamps. For example, such circuits may be found in the electronic ballasts for fluorescent lamps and for the more recently-developed solenoidal electric field (SEF) lamps. In such lamps, it is important that the ballast weight and cost be kept low. Conventional low pass filters are at least somewhat deficient in this respect. Additionally, since such lamps may be employed in a large number of locations, there is even further motivation to ensure a low level of electromagnetic interference.
Various forms of low pass filters are well known in the electrical and electronic arts. In particular, filters comprising inductor-capacitor ladder networks are well known. These ladder networks typically employ inductors as series elements and capacitors as shunt elements. The design cut-off frequency and frequency response characteristics of such filters are well known. In these ladder network low pass filters, the degree of high frequency attenuation desired is particularly controlled by the number of stages in the filter. However, in such filter arrangements it is necessary to provide a separate inductor for each stage. This not only adds weight but also cost to the filter.
The use of integrated inductor-transformers in a discharge lamp ballast circuit has also been well known in the past. In particular, it has been known that the use of a gapped leg in a multi-legged transformer core can effectively operate as a current-limiting inductor in an electronic gas discharge lamp ballast circuit. (See, for example, "Electric Discharge Lamps" by J. F. Weymouth, MIT Press, 1971), page 318. This concept is also disclosed in U.S. Pat. No. 4,187,450, issued Feb. 5, 1980 to DeYu Chen.
Additionally, it is to be noted that so-called constant voltage transformers have been employed in the past. However, these devices are described as including a capacitor connected across a transformer winding in such a way that the magnetic flux passing through this winding is the same as the flux passing through one of the transformer main (i.e., primary or secondary) windings. These capacitors therefore function as if they were connected across these main windings. The effective capacitance of such a capacitor is, of course, dependent on the turns ratio between the capacitor's winding and the main winding having the same magnetic flux.